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HDR on set

Somewhat silly question. I have an upcoming job where the DP might want to monitor HDR. I have read up on it on the last year, but some aspects are still not clear to me.

Both Alexa LF and DXL2 have a 6G-SDI output. That can do 2160p at 24 or up to 30. I assume that can be HDR or SDR. But if we stay 1080p24, can we still have HDR on an HD-SDI or 3G-SDI monitoring system? Let’s assume 4K HDR monitors, but I am hoping we don’t need to change over all the transmitters, matrixes and amps.

Does HDR necessaries Rec2020? Or can it exist within P3 or Rec709? We certainly won’t monitor on anything that comes even close to covering the 2020 color triangle.

And lastly: can I just feed this signal into sdr equipment and expect some results, or it won’t even display and I need an FS1 or similar to do the HDR-SDR conversion?

I have played with both the LF’s and DXL’s simulators but there is not much there. You can select the LF to output either 6G or 1.5G (no 3G?) and color space either 709 or 2020, but no indication (to me) if 2020 is automatically HDR. DXL has a preset called HDR2084 which I assume is what I need. But that has a separate 709/2020/redcolor setting.

Can’t help you with the DXL2 of course, and you have some very general questions about possible combinations of elements of standards that I’m going to leave alone here, but...

…yes, you can take the output of an ALEXA LF, apply an HDR look, and send that to a HD-resolution MON OUT that’s ben set up for Rec. 2020. That the signal is encoded with Rec. 2020 primaries in no way implies that the device needs to support the
full set of chromaticities that those primaries would imply. Short of a laser projector, or some new device with which I’m not familiar, you’re not going to find anything that supports all the chromaticities in Rec. 2020.

The ALEXA LF simulator at arri.com (Tools menu) isn’t showing the HDR LUTs, as you say, but I think that’s a bug. I will pass your observation back to the simulator and web teams. Anyway, walking down to
service and looking at an LF being checked out, there’s one LUT for HLG and one LUT for PQ already installed, named

ARRI 2100 HLG 1K 200.aml

ARRI 2100 PQ 1K 200.aml

Basically (assuming you have a 2020-configured PQ HD-resolution monitor connected to MON OUT by an HD-SDI cable):

Color from the ALEXA main menu (lower center button)

EVF/MON PROC, then MON 1 PROC to “Look File”

Select the built-in PQ look…

…and this will automatically set your MON 1 OUT to Rec. 2020, with the message “EVF processing changed to ALEXA Classic 709. MON OUTs with ‘Look’ changed to Rec. 2020."

There are LUTs in there for cinema grading, for monitor grading, etc etc but you would want “ARRI_Lookfiles_for_cameras”. At that point you have eight choices, two of which you already saw above:

ARRI 2100 HLG 1K 100.aml

ARRI 2100 HLG 1K 200.aml

ARRI 2100 PQ 1K 100.aml

ARRI 2100 PQ 1K 200.aml

ARRI 2100 PQ 2K 100.aml

ARRI 2100 PQ 2K 200.aml

ARRI 2100 PQ 4K 100.aml

ARRI 2100 PQ 4K 200.aml

These all assume a REC 2020-accepting monitor at the end of an HD-SDI cable from one of the MON OUTs. Let’s stick with PQ. Then the “1K”, “2K”, and “4K” help you narrow down the set of LUTs to be appropriate to your HDR monitor’s peak white (in
nits [i.e. candela/m^2]). And the “100” vs. “200” variants are about where your reproduction of diffuse white lands — again, in nits.

Anyway, with the look auto-setting the corresponding MON OUT, it’s kind of the reverse of your “2020 is automatically HDR”: with those HDR looks, the corresponding monitor output is automatically 2020.

...But if we stay 1080p24, can we still have HDR on an HD-SDI or 3G-SDI monitoring system? Let’s assume 4K HDR monitors, but I am hoping we don’t need to change over all the transmitters, matrixes and amps.

Does HDR necessaries Rec2020? Or can it exist within P3 or Rec709? We certainly won’t monitor on anything that comes even close to covering the 2020 color triangle.

...

I have played with both the LF’s and DXL’s simulators but there is not much there. You can select the LF to output either 6G or 1.5G (no 3G?) and color space either 709 or 2020, but no indication (to me) if 2020 is automatically HDR. DXL has a preset called
HDR2084 which I assume is what I need. But that has a separate 709/2020/redcolor setting.

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